Figures 22-24:
Various magnifications show "cannon-ball" masses of tumor cells. The key to diagnosing mesothelioma is not identifying a second malignant cell population. Final determination may require immunocytochemistry or a cell block with immunohistochemistry, electron microscopy, or other specialized techniques.

Tumor cells may be seen in a background of blood and proteinaceous debris. Groups of more than 12 cells may be a feature of malignancy.

Figure 27Pleural effusion Mesothelioma. 60X

Figure 27Pleural effusion:
Mesothelioma.
60X

At higher magnification, the high N/C ratio with variability in nuclear size and occasional multi-nucleation confirm the malignant nature of these cells. Differential diagnoses include adenocarcinoma and mesothelioma. Fine microscopic features of peripheral cell membranes and intercellular windows may suggest mesothelioma.

Figure 28Pleural effusion Mesothelioma. 60X

Figure 28
Pleural effusion:
Mesothelioma.
60X

Abnormal mitotic figures may be noted with mesothelioma, other malignancies, as well as occasional reactive mesothelial cells in effusions.